The present invention relates to a resin composition consisting of 5 to 95% by weight of vinylidene chloride resin (hereinafter referred to as PVDC) and 5 to 95% by weight of a polyamide resin of low melting temperature with a crystalline melting point of not higher than 210.degree. C. and to a molded product excellent in gas-barrier property and impact strength at low temperature comprising the above-mentioned composition.
PVDC is a resin which has excellent gas-barrier property and can be processed into films or bottles excellent in heat-sealing property, heat shrinkability and transparency, and is suitable for food-packaging. However, in the case of using PVDC only, the impact strength thereof at low temperature is small, and accordingly, several methods have been proposed for improving the mechanical strength of PVDC at low temperature.
For instance, a method of combining a liquid plasticizer with PVDC or a method of laminating PVDC and polyolefin, etc. has been carried out. However, the addition of a large amount of the liquid plasticizer makes the gas-barrier property poor and there is a possibility of permeating of the liquid plasticizer to the foods, and for producing the laminated molded products, it is necessary to provide a plurality of extrudes and the operation becomes complicated.
In order to reduce the amount used of the plasticizer, there is a technique of blending a chlorinated polymer with PVDC [refer to Japanese patent applications Laid-Open Nos. 53-58553 (1978), 53-136054 (1978) and 54-74848 (1979)]. Although the above-mentioned technique can prevent the deterioration of the gas-barrier property, the improvement of the impact strength at low temperature is not sufficient.
Moreover, the improvement of the impact strength of PVDC at low temperature is carried out by mixing a small amount of rubbery or elastic substance with PVDC [refer to Japanese Patent Publication No. 40-16141 (1965)]. However, there are cases where the molded product obtained by this method can not fulfill the impact strength at low temperature and the oxygen-barrier property at the same time.
On the other hand, polyamide resin is industrially utilized in the field of the melt-extruding molded products because of the excellent toughness, heat-resistance, cold-resistance, oil-resistance, transparency, etc. thereof, as the resin for single film, sheet or the resin for forming a layer of making laminated products (EPO139,888, GB2,035,198). However, since polyamide resin shows a large oxygen-Permeability coefficient and water vapor transmission rate at a high humidity, the range of utilization of polyamide resin has been limited.
Since polyamide resin is a structurally hydrophilic resin, it absorbs moisture in the atmosphere of high humidity, and it is plasticized by the thus absorbed water molecules resulting in the deterioration of the barrier property to oxygen, water, etc.
In the case where there is a moisture-dependency of the barrier property, not only the humidity of the outside atmosphere but also the water content of the packaged product itself must be considered, and accordingly, in the case of packaging the material containing a large amount of water, the gas-barrier property of polyamide resin becomes insufficient.
In general, to reduce the oxygen-permeability coefficient of polyamide resin, there is a method of mixing a saponified product of ethylene-vinylacetate copolymer [refer to Japanese patent application Laid-Open No. 62-7761 (1987)].
However, the oxygen-permeability under high moisture is still large, and there are problems of the occurrence of cross-linked gels at the time of melt-extruding, etc.
It has been desired to improve the hitherto-known defects of polyamide resin, namely (1) poor oxygen-barrier property under the atmosphere of high humidity and (2) large water vapor transmission rate under the atmosphere of high humidity, together with the problems of inability of maintaining the moldability, the gas-barrier property and the impact strength at low temperature in the case of mixing the polyamide with another resin for dissolving the above-mentioned defects, to a practically sufficient level at the same time.
As mentioned above, although PVDC and polyamide resin are respectively excellent resins, each of them has its own defects and a resin provided with both a practical gas-barrier property and an excellent impact strength at low temperature has been desired particularly in the field of food-packaging materials.
In order to solve the above-mentioned problem, a method of laminating a layer of PVDC and a layer of polyamide resin has been proposed [U.S. Pat. No. 4,112,182 and Japanese patent application Laid-Open No. 62-273849 (1987)]. However, since the adhesive power of each other of them is not sufficient, it is necessary to use a special adhesive and using a plurality of extrudes has a weak point of complicated operation.
As a result of the present inventors' earnest studies for obtaining a resin composition excellent in gas-barrier property and impact strength at low temperature and well balanced, they have found that a resin composition obtained by mixing PVDC and a polyamide resin having a crystalline melting point of not higher than 210.degree. C. has an excellent impact strength at low temperature and practical gas-barrier property, and on the basis of this finding, the present inventors have completed the present invention.
Further, the gas-barrier property in the present invention means the barrier property to oxygen and water vapour.